Flat panels for radiation imaging have been extensively studied for over ten years, and are well known in the art. Examples of flat panels for radiation imaging can be found in the following patents:
U.S. Pat. Nos. 5,132,541, 5,184,018, 5,396,072 and 5,315,101 assigned to Philips;
U.S. Pat. Nos. 4,785,186 and 5,017,989 assigned to Xerox;
U.S. Pat. Nos. 4,382,187, 4,799,094, 4,810,881, and 4,945,243 assigned to Thomson-CSF;
U.S. Pat. Nos. 5,182,624, 5,254,480, 5,368,882, 5,420,454, 5,436,458 and 5,444,756 assigned to 3M;
U.S. Pat. Nos. 5,079,426 and 5,262,649 assigned to Michigan University;
U.S. Pat. Nos. 5,340,988, 5,399,884, 5,480,810, 5,480,812 and 5,187,369 assigned to General Electric; and
U.S. Pat. No. 5,315,102 assigned to Fuji Xerox.
One type of flat panel radiation imaging system includes a thick amorphous selenium (a-Se) film on an array of pixels such as that described in the article entitled "Flat Panel Detector for Digital Radiology Using Active Matrix readout of Amorphous Selenium," by W. Zhao et al., Medical Imaging 96, SPIE Conference, SPIE 2708, February 1996. In this flat panel radiation imaging system, the pixels are arranged in rows and columns with each pixel including a TFT switch. Gate lines interconnect the TFT switches in each row of the array while source or data lines interconnect the TFT switches in each column of the array. The thick amorphous selenium film is deposited directly on top of the TFT switch array and a top electrode overlies the amorphous selenium film.
When x-rays are incident on the amorphous selenium film and the top electrode is biased with a high voltage, electron-hole pairs are separated by the electric field across the thickness of the amorphous selenium film. The holes, which are driven by the electric field, move toward the pixel electrodes (i.e. the drain electrodes of the TFT switches) and accumulate in a storage capacitor in each pixel. This results in a charge being held by the pixel electrodes which can be used to develop an x-ray image.
The charges held by the pixel electrodes are read on a row-by-row basis by supplying gating pulses to each gate line in succession. When a gating pulse is supplied to a gate line, the TFT switches of the pixels in the row associated with that gate line turn on, allowing the signal charges stored in the storage capacitor of those pixels to flow to the source lines. Ideally, the TFT switches of the array should be controlled only by the potential voltage on the gate electrode. However, stray electric fields from the amorphous selenium film and the top electrode, which can be up to 10V/m, can have significant effects on the channel conductance of the TFT switches unless special shielding techniques are used. One such shielding technique is to provide a dual-gate structure in the TFT switches. In these TFT switches, one gate is disposed below the semiconductor channel layer and the other gate is positioned above the semiconductor channel layer. The two gates are electrically connected together. An example of a dual-gate TFT switch is disclosed in "IEEE Transactions on Electronic Devices-28, No.6, pp.740-743, June 1981" by F. C. Luo et al.
Also, in medical x-ray imaging systems, signal levels are generally much lower than visible light imaging systems, in order to minimize the exposure of patients to x-rays. Therefore, in order to obtain high resolution, a high signal to noise ratio is extremely important. In order to improve the signal to noise ratio in x-ray imaging systems, amplified imaging pixels for flat panels have been considered such as those described in the "IEEE Journal of Solid-State Circuits, Vol. SC-4, No.6, pp. 333-342, December 1969" by S.G. Chamberlain and in the "Proceedings of IEDM'93, pp 575-578, December 1993" by H. Kawashima et al.
In order to reduce the switch noise caused by parasitic capacitance distributed along the source lines and maximize the signal to noise ratio, a charge amplifier is provided for each column of TFT switches in the pixel array. The charge amplifiers sense the charges on the source lines when a row of pixels is gated and provide output voltage signals proportional to the charges and hence, proportional to the exposure of the pixels to radiation. Unfortunately, by providing a charge amplifier for each source line, two problems result. Firstly, in large format radiation imaging systems which include in excess of one thousand (1000) source lines, the cost associated with the charge amplifiers is significant. Secondly, in high resolution radiation imaging systems that have a small pixel pitch, it is difficult to wire-bond the charge amplifiers to each source line. Accordingly, there is a need for an improved high resolution flat panel for radiation imaging.
It is therefore an object of the present invention to provide a novel high resolution flat panel for radiation imaging and a compensation circuit for an amplified flat panel which obviates or mitigates at least one of the above-mentioned problems.